Tao et al. Nanoscale Research Letters 2013, 8:296
http://www.nanoscalereslett.com/content/8/1/296
NANO EXPRESS
Open Access
A facile approach to a silver conductive ink with
high performance for macroelectronics
Yu Tao1*, Yuxiao Tao1, Biaobing Wang1, Liuyang Wang1 and Yanlong Tai2*
Abstract
An unusual kind of transparent and high-efficiency organic silver conductive ink (OSC ink) was synthesized with
silver acetate as silver carrier, ethanolamine as additive, and different kinds of aldehyde-based materials as reduction
agents and was characterized by using a thermogravimetric analyzer, X-ray diffraction, a scanning electron
microscope, and a four-point probe. The results show that different reduction agents all have an important
influence on the conductive properties of the ink through a series of complex chemical reactions, and especially
when formic acid or dimethylformamide was used as the reduction agent and sintered at 120°C for 30 s, the
resistivity can be lowered to 6 to 9 μΩ·cm. Furthermore, formula mechanism, conductive properties, temperature,
and dynamic fatigue properties were investigated systematically, and the feasibility of the OSC ink was also verified
through the preparation of an antenna pattern.
Keywords: Organic silver conductive ink (OSC ink), Conductive pattern, Formula mechanism, Antenna pattern
Background
Most research efforts in macroelectronics have opened
the door for the manufacture of lightweight, flexible,
cost-effective electronic devices that are beyond the
conventional silicon-based devices, including flexible
displays [1], flexible and conformal antenna arrays [2],
electronic solar cell arrays [3], radio-frequency identification tags [4], flexible batteries [5], electronic circuits
fabricated in clothing [6], and biomedical devices [7].
Usually, most of them require electrical contacts.
Up to now, various materials such as conjugated polymers, graphene, carbon nanotubes, and metals have
been used for the preparation of electrodes and conductive patterns using solution processing methods [8-11].
Specifically, metal nanoparticle inks have attracted more
and more attention due to their high conductivity and
thermal stability after having been sintered [12-14].
However, metallic nanoparticle inks often require
high annealing temperatures (>150°C) to decompose
stabilizing agents and other polymeric additives that
inhibit electrical conductivity, with the high annealing
* Correspondence: [email protected]; [email protected]
1
School of Materials Science and Engineering, Changzhou University,
Changzhou 201326, People’s Republic of China
2
Department of Biomedical Engineering, University of California Davis, Davis,
CA 95616, USA
temperature limiting the choice of substrate. Besides,
they still cannot completely avoid the condensation and
agglomeration of nanoparticles, especially after longterm storage. The agglomerated particles may damage
the equipment and influence the printing quality. During
preparation, a high-speed centrifuge or vacuum dryer
must be used to take nanometal particles out, so these
inks cannot be produced on a large scale. All of these
will cause a higher production cost [15-18].
There is no surprise to the fact that organic silver conductive ink (OSC ink) has received increasing attention as
a potentially much lower cost alternative [19-21]. This kind
of ink mainly consists of a silver carrier, weak reduction
agent, solvent, and additives, and a continuous conductive
silver track can be fabricated during the sintering process.
This strategy can compensate for the lack of conductive
metal nanoink and thus becomes the development direction of conductive ink for macroelectronics [22-25].
In our previous research, the relationship between
different kinds of amines and ink properties was investigated systematically. The addition of different amines
not only increased the solid content of the conductive
ink but also decreased the sintering temperature by
complexation [26-28].
Here, based on the previous results, the formula of the
conductive ink will be further optimized using silver
© 2013 Tao et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction
in any medium, provided the original work is properly cited.
Tao et al. Nanoscale Research Letters 2013, 8:296
http://www.nanoscalereslett.com/content/8/1/296
acetate as silver carrier, ethanolamine as additive, and
different kinds of organic aldehyde as reduction agents,
such as ethylene glycol, acetaldehyde, formic acid,
dimethylformamide, and glucose. Furthermore, the formula mechanism, conductive properties, temperature,
dynamic fatigue properties, and feasibility verification of
the OSC ink through the preparation of an antenna
pattern were also investigated systematically [29-31].
Methods
Materials
Silver acetate was obtained from Shanghai Lingfeng
Chemical Reagent Co., Ltd. (Shanghai, China). Polydimethylsiloxane (PDMS) including base and curing
agents was obtained from Dow Corning Co. (Midland,
MI, USA; SYLGARD 184 silicone elastomer). Polyester
film (0.1 ± 0.02 mm) came from Shanghai Weifen Industry Co., Ltd (Shanghai, China). Ethylene glycol, acetaldehyde, formic acid, dimethylformamide, glucose, ethyl
alcohol, and other solvents were of analytical grade and
used without further purification. Deionized water was
used in all experimental processes.
Synthesis of OSC ink
For the preparation of conductive ink (1 g), silver acetate
(0.32 g; which means if all silver ions are reduced to
elemental silver, the content of elemental silver is 20 wt.%)
and ethanolamine (0.2 g) were added to ethanol (0.13 g)
and different reduction agents (0.35 g; ethylene glycol,
acetaldehyde, formic acid, dimethylformamide, or glucose, etc.) under vigorous stirring until a transparent
solution was obtained.
Preparation of antenna pattern
For the preparation of the PDMS pattern as template,
polyethylene terephthalate (PET) was adhered to a sheet
glass using both side tapes, and 3-g PDMS (base/curing
agent is 15/1) was dropped on the center of the PET
film. Then, after spin coating (800 rpm), baking at 80°C
for 3 h, and laser etching, the desired PDMS pattern as
template can be fabricated with the conductive track (a
thickness of 200 μm and a width of 200 μm).
For the preparation of the antenna pattern, the synthesized OSC ink was dropped into the trench of the PDMS
template track using a syringe, and the ink will flow to
all of the track spontaneously until full; then, it will be
sintered at 120°C for 30 s. Finally, the PDMS template
can be peeled off easily by forceps, and the desired antenna pattern was achieved [32].
Instrumentation
OSC ink was characterized by using a Ubbelohde viscometer (CN60M, Zxyd Technology Co., Ltd., Beijing,
China); a surface tension instrument (A101, Kino
Page 2 of 6
Industry Co., Ltd, New York, USA); X-ray diffraction
(XRD; max 2550 PC, Rigaku-D, Rigaku Corporation,
Tokyo, Japan) using Cu Kα radiation; scanning electron microscopy (SEM; S-360, Cambridge Instruments Ltd., Cambridge, England) operated at 10 kV;
thermogravimetric analysis (TGA; QS-500, TA Instruments Inc., New Castle, DE, USA) with a heating
rate of 5°C·min−1 in a nitrogen atmosphere; a fourpoint probe equipped with a semiconductor characterization system (BD-90, Shanghai Power Tool Institute,
Shanghai, China); a memory hicorder (8870–20, HIOKI,
Nagano, Japan) with a heating device from room temperature to 120°C, a steady current mechanism (10 mA),
and an amplifier (×100); and a dynamic fatigue tester
(built in the lab) with steady current mechanism (10 mA).
The antenna pattern was investigated using a Uscan
explorer with 3D profilometer system (D46047, Nanofocus,
Oberhausen, Germany).
Results and discussion
Formula mechanism
Compared with nanosilver conductive ink, the synthesized silver organic ink is transparent and clear without
any visible particles. During the preparation process, this
kind of conductive ink was mainly composed of a silver
carrier, weak reduction agent, solvent, and additives. At
the room temperature, it was very stable and can be kept
for at least 1 month. Once it was heated, the complex
chemical reaction occurred between the various components. Generally speaking, the sintering process can be
divided into four stages: firstly, from simple silver ion to
silver ion complex, then to silver oxide, and finally to
elemental silver. Meanwhile, the color also changes from
colorless to faint yellowish brown, to black, and to
metallic luster. The details can be seen from Figure 1
directly.
In this formula, silver acetate was chosen as silver
carrier, which can control the reaction rate effectively by
adjusting the concentration of the silver ion in the
mixing solvent because of its worse solubility. Ethanolamine was used to increase the silver content of the
conductive ink to guarantee the conductivity and further
to decrease the sintering temperature.
Different aldehyde-based materials were chosen as
weaker reduction agents, which have been discussed in
detail as shown in Figure 2. Generally speaking, such
materials can be divided into two types: one for itself
with the aldehyde group, such as acetaldehyde, formic
acid, dimethylformamide, and glucose; another for itself
without the aldehyde group, but after heating, the aldehyde group can appear, such as ethylene glycol which
can change to acetaldehyde at a high temperature and
glycolic acid which can be decomposed into formaldehyde, carbon monoxide, and water at 100°C. The results
Tao et al. Nanoscale Research Letters 2013, 8:296
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Figure 1 Scheme of chemical reaction mechanism of OSC ink. R0, R1, and R2 are carbon chains.
show that reduction agent plays an important role on
the properties of the conductive ink. Usually, a stronger
reduction agent will bring in the instability of the ink,
leading to the precipitation of silver particles and lower
conductivity. Conversely, a weak reduction agent will
result in a higher sintering temperature. It can be inferred that a suitable reduction agent is very important
to get lower resistivity. From Figure 2, at the sintering
temperature of 120°C for 1 h, the resisitivity of the silver
thin film with different formulas should be very stable. It
can be seen that formic acid and dimethylformamide
show lower resistivity of about 6 to 8 μΩ·cm and 7 to 9
μΩ·cm, respectively. In view of the formula stability, in
the following research, dimethylformamide was chosen
as the reduction agent.
OSC ink properties
For further investigation of the OSC ink, dimethylformamide
was used as reduction agent in the formula. The viscosity
and surface tension were adjusted to 13.8 mPa·s and 36.9
mN/m (20°C), which can totally fulfill the requirement of
ink-jet printing, as shown in the inset of Figure 3a.
The thermal properties of the prepared OSC ink were
investigated by TGA with a heating rate of 5°C/min, as
depicted in Figure 3a. It can be seen that there exists
an evident mass-decreasing area, from 80°C to 160°C,
which is related to the evaporation of organic materials;
finally, 20.3 wt.% of the mass remains, which indicates
that the ink contains 20.3 wt.% silver and agrees well
with the calculated value (20 wt.%). If several drops of
ammonia were added, the solid content can be further
increased to 28 wt.% at most because of its stronger
coordination ability than ethanolamine. However, more
ammonia will cause the instability of the conductive ink
due to its volatilization.
The conductive properties of the prepared OSC ink
were investigated using different sintering temperatures
(90°C, 120°C, 150°C) for different durations of time
(from 0 to 60 min), which also can be explained by percolation theory, as shown in Figure 3b. During the
sintering process, initially, there are only silver acetate
and silver oxide, without any elemental silver, so there is
no conductivity. Then, almost all of the silver oxide was
reduced to elemental silver at the same time, indicating
that a continuous conductive track has been fabricated
and showing metallic luster and high conductivity. Especially, based on the present formula of the ink, when the
sintering temperature is 120°C for 30 s, the resistivity
can drop to 7 to 9 μΩ·cm.
Figure 3c shows an XRD pattern of the silver ink after
sintering, and all diffraction peaks could be indexed to
the face-centered cubic phase of silver. The lattice constant calculated from this XRD pattern was 4.098, which
was very close to the reported data (a = 4.0862, JCPDS
file no. 04–0783). The inset is the surface morphology of
the conductive ink after sintering, and more information
also can be seen from Figure 3d.
Temperature and dynamic fatigue properties
Figure 2 Resistivity of OSC ink (20 wt.%) with different
reduction agents sintered at 120°C for 1 h.
To verify the applicability of this approach in macroelectronics, the correlations between resistivity and
temperature, and dynamic fatigue of the conductive silver
Tao et al. Nanoscale Research Letters 2013, 8:296
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Figure 3 Ink properties. (a) TGA and DTG curves (inset, OSC ink). (b) Variation of resistivity sintered at different temperatures for different times.
(c) XRD pattern of sintered OSC ink with a solid content of 20 wt.% (the inset shows the top-view SEM image of the conductive film). (d) Lateral
view of the SEM image of the silver film sintered at 120°C for 30 s (dimethylformamide was used as reduction agent in the formula).
Figure 4 Correlations between resistivity and temperature, and dynamic fatigue of the conductive silver line. (a) Relationship and (b)
measurement equipment of resistance versus the change of the temperature. (c) Dynamic fatigue properties of PET-based conductive patterns
sintered at 120°C for 30 s.
Tao et al. Nanoscale Research Letters 2013, 8:296
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line were investigated systematically, which were shown in
Figure 4.
From Figure 4a,b, a set of equipment including a
heating device from room temperature to 120°C, steady
current mechanism (10 mA), amplifier (×100), memory
hicorder (HIOKI, 8870–20), etc. were assembled together, aiming at monitoring the changes of the resistivity of the conductive silver line during the heating and
cooling processes. It can be obtained that between 20°C
and 100°C, the largest variable quantity of the resistivity
is just about 0.28 Ω. After linear fitting, the slopes of the
heating curve and the cooling curve, which can be called
temperature coefficient of resistance (TCR), approximately have the same slope (kh = kc = 0.0007 aR/°C−1),
indicating the good thermal stability of the conductive
silver line. The TCR is a little different compared
with the TCR of bulk silver (0.0038 aR/°C−1). This
phenomenon is mainly caused by the complex microstructure of the silver thin film which will bring more
barriers during the electron-transfer process. Moreover,
it also can be seen that though the heating curve and
cooling curve have the same TCR, the cooling curve is
always below the heating curve. This is mainly because
the natural cooling process (about 28 min) needs more
time than the heating process (15 min).
From Figure 4c, a bending tester was used to study
the dynamic fatigue of the PET-based conductive silver
line. During the test, the conductive line makes a periodic bending movement from I to V, and every period
needs 2 s. The details also can be seen from the set in
Figure 3b. It is very interesting to find that the resistivity of the conductive silver lines also increases with
the increase of the bending angle. From I to III, the
resistivity increases from 5.2 to 5.76 Ω. It can be
explained that when bending, the silver thin film was
stretched and became thin, especially on the top point
of the conductive line, so the stack density and conductivity decreased. From III to V, the resistivity was
back to 5.2 Ω, and after a periodic movement like this for
1,000 times, the resistivity did not significantly increase
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due to the good ductility of the metal silver. Generally
speaking, compared with other printing technologies, this
method also shows good adhesion between the silver thin
film and PET, showing good results.
Preparation of an antenna pattern
To test the practical applications of the prepared OSC
ink here, an antenna pattern (11 mm × 12 mm) was
designed and fabricated using fit-to-flow or drop
method, which also can be seen from Figure 5 directly.
The thin film PDMS pattern with a thickness of 200
μm, a width of 200 μm, and a total length of 15.8 cm on
the PET substrate was prepared using a laser and used
as template. The synthesized OSC ink with blue dye
(seen more clearly) was dropped to the center of the
template using a syringe (20 μL per drop). Due to the
good wetting and film-forming ability of the ink, it will
flow along the template track until it fills the whole
track, especially after plasma treatment with oxygen.
After sintering at 120°C for 30 s, the continuous conductive track can be fabricated, and the total resistor Rab
decreased to 4.6 Ω measured by a multimeter (middle
image of Figure 5) with a width of 200 μm and thickness
of 22 μm according to the surface profile.
Conclusions
In summary, an unusual kind of high-efficiency, transparent organic silver conductive ink (OSC ink) was
synthesized with silver acetate as silver carrier, ethanolamine as additive, and different kinds of aldehyde-based
materials as reduction agents successfully. The results
show that different reduction agents have an important
influence on the ink properties through a series of
complex chemical reactions, and when formic acid or
dimethylformamide was used as the reduction agent and
sintered at 120°C for 30 s, the resistivity can be lowered
down to 6 to 9 μΩ·cm. It also can be obtained that the
fabricated conductive pattern shows good temperature
and dynamic fatigue properties. Besides, the feasibility of
the synthesized OSC ink was verified through the
Figure 5 Antenna pattern after sintering at 120°C for 30 s and surface profile curves of conductive pattern. The prepared antenna
pattern was fabricated using drop or fit-to-flow method.
Tao et al. Nanoscale Research Letters 2013, 8:296
http://www.nanoscalereslett.com/content/8/1/296
preparation of an antenna pattern using drop or
fit-to-flow method successfully.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
Y-LT synthesized the organic silver conductive ink and discussed the formula.
YT, LW, YT, and BW characterized and investigated the properties of the OSC
ink. All authors took part in the writing of the manuscript and approved the
final manuscript.
Acknowledgments
This work was supported by the project funded by the Priority Academic
Program Development of Jiangsu Higher Education Institutions (PAPD).
Received: 22 March 2013 Accepted: 23 May 2013
Published: 25 June 2013
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doi:10.1186/1556-276X-8-296
Cite this article as: Tao et al.: A facile approach to a silver conductive
ink with high performance for macroelectronics. Nanoscale Research
Letters 2013 8:296.
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